In a semiconductor device, a tungsten (W) is used as a wiring material of a wiring configuration or a material of an interdiffusion barrier. In this case, a W film is formed to fill in a contact hole or a via hole formed in an interlayer insulating film on a semiconductor wafer, which is a substrate to be processed, or to coat an inner surface thereof.
So far, PVD (Physical Vapor Deposition), typically a sputtering, has been used as a method for forming the W film. However, with PVD, it is difficult to handle a high coverage required under a recent trend toward a miniaturation in a device. For this reason, recently, CVD capable of dealing with such a trend has been employed as a method for forming the W film.
For example, as a processing gas for forming the W film in CVD, a tungsten hexafluoride (WF6) and a reducing gas, an H2 gas are used. In this case, the processing gases react like the following reaction equation, to thereby form the W film on the wafer.
[Reaction Equation]WF6+3H2→W+6HF
However, since a design rule is getting more and more strict, F has a bad influence on a film quality of a gate oxide film in case of using such an F containing gas, resulting in a device trouble.
Meanwhile, references 1, 2, and 3 disclose that a tungsten carbonyl (W(CO)6) of an organic compound gas is used as a processing gas not containing F, in order to form the W film by using CVD. Further, reference 5 describes that a metal film is formed by CVD using a metal carbonyl like W(CO)6. Still further, reference 4 discloses that the W film is formed in the semiconductor device by CVD using W(CO)6. The metal carbonyl such as W(CO)6 does not have a bad influence on the device unlike WF6, which cause a bad influence on the device due to F, so that it has bright prospects as a source gas of CVD.
Reference 1: Japanese Patent Laid-Open Publication No. Hei 2-225670
Reference 2: Japanese Patent Laid-Open Publication No. Hei 4-173976
Reference 3: Japanese Patent Laid-Open Publication No. Hei 4-27136
Reference 4: Japanese Patent Laid-Open Publication No. 2002-124488
Reference 5: J. Vac. Sc1. Technol., 14(2), March/April, 1996, pp. 415-424
However, there is a problem in using the metal carbonyl like W(CO)6. For instance, CO, which is produced by decomposition of the metal carbonyl gas like W(CO)6, has a strong affinity for a metal. Accordingly CO is adsorbed onto a surface of the substrate to be introduced into the metal film as an impurity. As a result, an electrical resistance of the metal film increases (see right column on p. 417 of reference 5). For the same reason, the metal carbonyl like W(CO)6 has not been put to practical use sufficiently, even if it is considered as a promising CVD source gas.